1. Field of the Invention
The invention relates to electric lamps and particularly to electric lamps. More particularly the invention is concerned with a PAR lamp with a lens having a pattern of refractive lenticules.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Reflector lamps provide controlled beam patterns with light distributions with beam spreads ranging from a narrow spot with a beam angle of 10 degrees or less to a wide flood with a beam angle greater than 45 degrees. Most applications for screw based lamps require a smooth beam with circular symmetry that is free of light and dark bands or striations. This is especially true for retail lighting where the customer can be very concerned with beam quality. Most of the useful light from a reflector lamp is within the beam angle defined as the total angular spread of a cone of light to 50 percent of maximum intensity. Although the shape and uniformity of the central beam are important, the uniformity of light outside the central beam can also be critical to illumination quality. Light striations in the outer region can be easily detected by the human eye and distracting.
Typical reflector lamps use a parabolic reflector to collimate the light and then a lens with a pattern of optical elements or lenticules to spread the beam to the desired shape. Alternately, a random stipple pattern on the lens inner surface can be used to provide a small spread angle suitable for the relatively narrow beam of a spot lamp. A problem with the most common lens lenticule patterns is that the beam is distorted from the desired circular shape and the regular lenticular pattern causes an objectionable pattern of light and dark bands in the illuminated region outside the central beam. U.S. Pat. No. 6,086,227 describes the problems with most typical reflector lamp optics and proposes a spiral lenticule optic pattern that substantially reduces the non-uniformity and beam striations in most cases.
FIG. 1 shows a prior art end view of a PAR lamp lens with a spiral lenticule pattern. The spiral pattern of the U.S. Pat. No. 6,086,227 design has been found to still have an underlying hexagonal symmetry that slightly distorts the desired circular beam shape. This hexagonal symmetry is especially noticeable in smaller spot size lamps such as a PAR20 using fewer lenticules. The hexagonal symmetry of the spiral pattern also contributes some striations that are typically not objectionable in flood lamps with beam angles wider than 25°.
The striations created by the spiral pattern are more obvious with narrow beam angles. The lens with spiral lenticule pattern scatters less light outside the central cone, but does project an unacceptable pattern of light and dark streaks. These striations can be more pronounced than those produced by hexagonal close packed lenticules. For this reason, most PAR lamps with beam angles less than 15 degrees use a random stippling on the lens inner surface to create desired spread. The stippled lenses have a relatively smooth circular beam with minimal striations.
Although stippled lenses provide acceptable performance, lenticules offer several advantages over lens stippling. For assured quality control of the beam, manufacturers prefer, if possible, to use defined lens optics in lamps with spread beams. The stippling on the glass pressing tool is usually created by one of these processes—acid etching, electrical discharge machining (EDM), or shot blasting. All these processes have inherent variations that are difficult to control, leading to wide beam angle tolerances. The stippled tooling also wears faster than machined lenticule tooling increasing costs. Most significantly, the machined lenticules offer greater control of beam spread than does a random stipple pattern, so more light can be directed into the useful beam with less light scattered outside the useful beam. Test results have confirmed that spot lenses using lenticule optics have 5 to 10 percent higher center beam candlepower than stippled lenses of equal beam angle providing a substantial performance improvement.
Effective lenticule patterns must have uniform lenticule spacing over the face of the lens to avoid deviation from circular beam shape and to maximize luminous flux within the beam angle. But any regular pattern should be avoided to minimize striations and beam distortion. A random lenticule pattern results in a wide distribution of lenticule spacing causing poor beam lumens. All known lenticule lens patterns used on PAR lamps have some regular pattern or rotational symmetry that adversely affects the light distribution. Lenticules are typically arranged in a hexagonal or square pattern.